Stocking Rate

Poplar plantation, NSW

The tree density or stocking rate of a forest is described as the number of trees per hectare. This can be easily calculated for each plot as follows:

Stocking rate (stems/ha) = Trees in plot / Plot area (ha)

For example: if there were 30 trees in a 20 x 15 metre rectangular plot then the stocking rate would be:

Stocking rate (stems/ha) = 30/[(20x15)/10000*)] = 30/0.03 = 1000 stems/ha

*Note that the plot area must be in hectares (hence the division by 10000)

If trees are established in a regular pattern and there have been no losses, then the stocking rate can be estimated from the initial spacing:

Stocking rate (stems/ha) = (100/Spacing between rows) x (100/Spacing within rows)

For example: if a plantation was established at a spacing of 4 by 5 metres then the stocking rate would be:

Stocking rate (stems/ha) = 100/4 x 100/5 = 25 x 20 = 500 stems/ha

In belts the stocking rate might be better described as the number of trees per 100 metres of belt.

If stocking rate is the only factor of interest it may not be necessary to establish fixed area plots. ‘Point-to-plant’ sampling is an effective and efficient technique for assessing stocking rate. This technique involves measuring the distance from an identified ‘plot point’ to the forth closest tree. To calculate the stocking rate for each plot the formula is:

Stocking rate (stems/ha) = (n - 0.5) x 10000/(3.142xd2)

Where:

n = number of trees in each stocking plot (4 if the distance to the forth tree was measured)
d = distance to the nth tree in metres

Therefore, if the distance to the 4th tree were 5 metres then the equivalent stocking rate would be:

Stocking rate (stems/ha) = (4 - 0.5) x 10000/(3.142x52) = 35000/78.55 = 446 stems/ha

Because stocking plots can be done quite quickly, it is possible to do many measurements systematically across the whole forest to get a precise measure of the stocking rate. This method can also be used to determine the variability in stocking rate across the site. This is important in determining the number of fixed area plots required for more detailed measurements such as diameter, height and volume.